German patent document no. 35 26 462 discusses a method for checking the air quality in motor vehicles using an internal and an external sensor in a moving or stationary vehicle. The fresh air flaps are either switched to circulating air or closed when a level of harmful substances detected by the external sensor is exceeded.
If the alarm is triggered, i.e., the level of harmful substances in the exterior air has exceeded a predefined value, the value of the interior sensor is logically compared with the value of the exterior sensor. If the air quality in the vehicle is poorer than the air quality outside the vehicle, the procedure is reversed, i.e., the flaps are switched to supply of exterior air.